Device for crucible-free or floating zone melting of a crystalline rod

ABSTRACT

IN DEVICE FOR FLOATING ZONE MELTING A CRYSTALLINE ROD INCLUDING A REMOVABLE HOLDER MOUNTED ON THE END OF A SUBSTANTIALLY VERTICAL HOLDER SHAFT FOR END-SUPPORTING A SUBSTANTIALLY VERTICAL CRYSTALLINE ROD COAXIAL TO THE HOLDER   SHAFT, THE IMPROVEMENT THEREIN COMPRISING SEPARATE MEANS FOR RESPECTIVELY PREVENTING RADIAL DISPLACEMENT OF THE END HOLDER AND ANGULAR DEVIATION THEREOF RELATIVE TO THE SHAFT.

July 13, 1971 R. sums DEVICE FOR CRUCIBLE-FREE 0R FLOATING ZONE MELTING OF A CRYSTALLINE ROD Filed Dec. 6 1967 2 Sheets-Sheet 1 July 13, 1971 DEVICE FOR CRUCIBLE OF A GRYS Filed Dec. 6 1967 EMEIS -FREE 0R FLOATING ZONE MEL'IING TALLINE ROD 2 Sheets-Sheet 2 .3 &////// 1 Fig. 3 4,

United States Patent 0 3,592,937 DEVICE FOR CRUClBLE-FREE OR FLOATING ZONE MELTING OF A CRYSTALLINE ROD Reimer Emeis, Ebermannstadt, Germany, assignor to Siemens Aktiengesellschaft, Berlin and Munich, Germany Filed Dec. 6, 1967, Ser. No. 688,605 Claims priority, applicatitan sfigrmany, Dec. 7, 1966,

Int. Cl. B0ld 9/00 US. Cl. 23-273 8 Claims ABSTRACT OF THE DISCLOSURE My invention relates to a device for crucible-free or floating-zone melting a monocrystalline, rod such as of semiconductor material, including a removable holder located at the end of a substantially vertical shaft for rotating the rod, the rod being disposed vertically and coaxially to the shaft.

In heretofore known devices of this general type, the crystalline rod is clamped at both ends thereof within respective holders. To fasten the rod within each holder, there are generally provided several fastening members, such as screws, clamping jaws or pivotable levers, mounted on the holder and disposed in two planes located transversely or substantially perpendicularly to the axis of the rod. Since the rod is thereby firmly fastened in the holders, the thermal stresses produced when heating the rod initially to form the melting zone and when cooling the rod after it recrystallizes or solidifies when leaving the melting zone must therefore be absorbed therefrom. As has been repeatedly established, the thermal stresses have a deleterious effect on the crystal structure of a crystalline rod, such as a semiconductor rod especially. The thermal stresses frequently cause a seed crystal fused to a rod end to break away upon cooling and, thereby, at least part of the crystalline rod becomes useless. On the other hand, when passing an incandescent zone through the crystalline rod, example, the thermal stresses produced thereby may be so great as to cause the rod to break apart.

It is accordingly an object of my invention to provide device for carrying out a crucible-free or floatingzone melting operation on a crystalline rod which avoids the aforementioned difficulties of the heretofore known devices of that type.

It is more specifically an object of my invention to provide holders in the aforementioned device that are constructed so that the length of the crystalline rod, on the one hand, and of the holder shaft, on the other hand, which is necessary for effecting a firm seating in the holder, is as short as possible, whereby for a predetermined spacing of the shafts for both end holders of the rod, the effectively usable pulled or recrystallized length of the crystalline rod is increased.

With the foregoing and other objects in view, I provide in accordance with my invention, device for crucible-free or floating zone melting of a crystalline rod which comprises end holders for a crystalline rod that is being processed, and separate guide means for respectively preventing radial displacement of an end holder and angular deviation thereof relative to the holder shaft. By providing separate means for preventing displacement of the end holder in the radial direction and for preventing angular deviation of. the end holder relative to the holder shaft, respectively, even when the seating of the part of the holder surrounding the rod is practically immovable in a direction radial to the rod axis, the part of the holder can be freely displaceable in a direction parallel to the rod axis. The separate guide means can be adjusted to one another so that the free displaceability of the holder portion surrounding the rod is ensured even when the individual parts of the holder are heated to different temperatures.

In accordance with additional features of my invention, the holder is prevented against radial movement by a central bolt, and against angular deviations by adjustable bearings lying in a plane that is transverse or substantially perpendicular to the holder shaft. The guide bolt and the adjustable bearings are so disposed that the freely displaceable part of the upper rod holder and of the lower rod holder are yieldable or flexible respectively in the direction of elongation and in the solidifying or recrystallizing direction.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the inventionis illustrated and described herein as embodided in device for crucible-free or floating zone melting of a crystalline rod, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIGS. 1 and 2 are longitudinal sectional views respectively of the upper and lower holders for a crystalline rod in accordance with my invention; and

FIG. 3 is a longitudinal view of a zone melting chamber showing in assembly the various components of the device of my invention, including the holders of FIGS. 1 and 2.

Referring now to the drawings, and first particularly to FIG. 1 thereof, there is shown the holder 1 for a crystalline rod 2. The holder 1 is made up of a fastener sleeve 3, a suspension device 4 with an inner flange 7, a flanged bushing 5 and a runner disc 6. The crystalline rod 2, preferably a silicon rod, is fastened within the fastener sleeve 3 by a plurality of fastening members, such as screws 8 and 9, mounted on the sleeve 3 and disposed in two separate planes extending transversely to the axis of the sleeve 3. The length of the crystalline rod 2 which is clamped within the sleeve 3 is relatively short, especially when compared to the overall length of the rod 2. The spacing A between both fastening planes is, for example, 2 cm. in length. The flanged bushing 5 is secured on a shaft 11 for the holder by a tightening or tensioning jaw 10. The holder shaft 11 is rotatable and is displaceable in the direction of the rod axis S. The crystalline rod 2 clamped in the fastening sleeve 3 is suspended from the flanged bushing 5. The device 4 for suspending the rod 2 and sleeve 3 from the bushing 5 includes a hollow cylinder 14 open at its top surface 13 which is closed by a releasably secured inner flange 7 which is braced in whole or in part on the flanged bushing 5. If the holder shaft 11 is not to be rotated, the suspension device 4 can, for example, be braced on the flanged bushing 5 by three claw-shaped flange parts. In the embodiment shown in FIG. 1, the inner flange 7 is in the form of an annular disc, the inner wall 16 of which is provided as a runner or race surface for the roller members, such as the balls 17, which serve as bearings for the holder 1. The balls are guided in an additional annular disc or ring 18, which serves as a damping member when the holder shaft 11 rotates freely. The axial dimensions of the hollow cylinder 14 are chosen so that in the even of an expansion or elongation of the crystalline rod 2, the inner flange 7 can be raised from the balls 17, i.e. the holder 1, except for the parts 5, 6, 17 and 18, is freely displaceable in the expansion or elongation direction of the crystalline rod 2. the race disc 6 is installable with the race 21 thereof disposed transversely or substantially perpendicularly to the holder shaft 11 by means of tension screws 19 and compression screws 20. To ensure the prevention of radial movement of the holder 1, a central bolt 22 is inserted in the base 23 of the hollow cylinder 14, and extends into a bore 12 formed in the flanged bushing 5. The fastening sleeve 3 is provided with a cover plate 24 which is braced with a pin 25 against the base 23 of the hollow cylinder 14. The cover plate 24 is tiltable by means of the set screws 26 about the pin 25. Transversely elongated bores 15, through which the set screws 26 extend, also permit a lateral displacement of the cover plate 24. Thereby, inaccuracies in the clamping of the crystalline rod 2 can be balanced or equallized, especially if the clamped portion of the crystalline rod 2 is not in a straight line with the rod axis.

As shown in FIG. 2, the lower rod holder 27 is made up of a ground plate 28, a base plate 29 and a fastening sleeve 30. The base plate 29 and the fastening sleeve 30 therewith as well as the crystalline rod 2 and a seed crystal 31 exert pressure of their own weight on adjustable bearings 32. The bearings 32 are preferably formed as spheres so that a point contact is provided. The bearings 32 are adjustable by means of set screws 33 which are threadedly inserted in the ground plate 28. Three set screws 33 are advantageously disposed on the periphery of the ground plate 28. A pin 34 inserted in the base plate 29 ensures against radial displacement of the lower holder 27. The holder shaft 35, like the holder shaft 11, is rotatable and can also be displaced in the direction of the rod axis S. Fastening members, such as screws 37 and 38, for example, are provided on the fastening sleeve 30 again disposed in two different planes, respectively, which firmly retain the seed crystal 31 in the fastening sleeve 30. The spacing B between the two fastening planes is about 2 to 4 cm. and is preferably shorter than or at most the same length as the radial spacing of the bearings 32 from the rod axis S. The base plate 29 has the appearance of a trough or tub with a peripheral raised wall 39. The base plate 29 thereby forms effective protection for the vacuum-tight seals 45, 45 of insulating material such as rubber for example, through which components of the device extend into the zone-melting chamber, as shown in FIG. 3, by preventing, for example, the molten semiconductor material from the melting zone 43 from dripping onto the seals. By means of the set screws 33, the bearing surface of the base plate 20 is adjusted substantially perpendicularly to the axis of the holder shaft 35.

It is, of course, readily apparent that individual features of the illustrated and described embodiments can be varied within the scope of and range of equivalents of the invention. For example, the radial distance of the bearings 17 and 32 from the rod axis S, which can be particularly for the lower rod holder 27 about one-fourth the length of the crystalline rod 2, can be increased greatly above this proportionate value. Such an increase of the bearing spacing also requires a larger ground plate 28 and base plate 29, however, but this has the advantage that the vacuum tight passage seals 45 in the base of the zone melting chamber are protected not only against the spraying or dripping of material from the melting zone, as aforementioned, but are widely protected, during a treatment process under vacuum, against vaporous precipitations of any type. It is also important that all race surfaces and bearing surfaces of both holders are widely protected against deposition of substances precipitating out of the melting zone. It is further of importance that the heat transfer from the crystalline rod 2 to the holder shafts l1 and 35 is greatly impeded, since the hot parts of the holder which are connected to the rod are in contact with the cold parts of the holder respectively only through a three-point bearing and through the guide pins 22 and 34. Thereby, liquid cooling of the holder shafts 11 and 15 can widely be dispensed with. It is moreover advantageous that the guiding means against radial displacement, i.e. the guide bolts or pins 22 and 34 are very narrow (2 to 3 mm), so that they expand or elongate only slightly when heated. Any possible binding occurring within the respective guide bushing is thereby practically eliminated. Since the bearings 17 and 32 are adjustable and, thereby, the location of the bearing surfaces of the base plate 29 and of the inner flange 7 substantially perpendicular to the holder shafts 11 and 35 can be produced independently of the accuracy of the machining of the holder shafts 11 and 35 and of the parts 6 and 28 serving as counter bearings, the clamped length of the holder shafts 11 and 35 within the members 5 and 28, respectively, can be kept very short, for example 1, to 2 cm. long. Thereby a further improvement in the effective recrystallized length of the crystalline rod 2 is obtained.

In FIG. 3, there is schematically shown a sectional view of a zone melting chamber 41 having a window 42 for observing the progress of the zone melting process therein. The drive shafts 11 and 35 for the respective rod portions 2:: and 2b of the semiconductor rod 2 extend through vacuum-tight seals 45, which may be oil seals, for example, into the zone melting chamber, which is either under vacuum or contains an inert gas atmosphere. An induction heating coil 44 for producing the melting zone 43 extends vacuum-tightly through a passage seal 45 located in a side wall of the zone melting chamber 41. Instead of providing a fixed point in the sidewall of the chamber 41 through which the heating coil 44 extends, the heating coil 44 can be suitably mounted so that it is reciprocatingly displaceable along the axis of the rod S, as shown for example in my Pats. Nos. 3,030,194 and 3,216,805.

I claim:

1. In device for floating-zone melting a crystalline rod including a removable holder mounted at an end of a vertical holder shaft for en-supporting a substantially vertical crystalline rod disposed substantially coaxially to the holder shaft at the lower end of the rod, the improvement therein comprising separate means for respectively preventing radial displacement of the lower end holder and angular deviation thereof relative the shaft, including a base plate secured to the lower end holder, said base plate having a flat surface resting on at least three spherical bearings, said bearings being connected with the vertical shaft for the lower end holder, said bearings being guidingly received in a ground plate secured to the lower holder shaft, and being adjustable in the axial direction of the lower holder shaft.

2. In device for floating-Zone melting a crystalline rod including a removable holder mounted at the end of a vertical holder shaft for end-supporting a substantially vertical crystalline rod disposed substantially coaxially to the holder shaft, the improvement therein comprising separate means for respectively preventing radial displacement of the end holder and angular deviation thereof relative to the shaft and including a flanged bushing secured to the holder shaft, the holder being the upper end for the rod and being suspended from said flanged bushing.

3. Device according to claim 1, including set screws inserted in said ground plate for adjusting said bearings.

4. Device according to claim 2, including a race discs mounted on said flanged bushing and having a race adjustable substantially perpendicularly to the upper holder shaft.

5. Device according to claim 2, including means for suspending the holder from said flanged bushing, said suspension means comprising a hollow cylinder open at its upper end, and having a removably fastened inner flange at said upper end thereof, said hollow cylinder being braced by at least parts of said inner flange against said flanged bushing.

6. Device according to claim 5, including rolling bearings located within said hollow cylinder, and wherein said inner flange has the shape of an annular disc, said flanged disc having an interior surface within said hollow cylinder serving as race surface for said bearings.

7. Device according to claim 2, including means for suspending the holder from said flanged bushing, and a clamping sleeve fastened to the upper holder and laterally displaceably mounted on said suspension means.

References Cited UNITED STATES PATENTS 6/1965 Hans 23-273 3/1966 Karstensen 23301 NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner US. Cl X.R. 82-45; 2796 

